Hard coating film

ABSTRACT

The present invention relates to a hard coating film, and, more particularly, to a hard coating film having hardness and excellent physical properties. According to the present invention, the hard coating film has high physical properties including hardness, scratch resistance, transparency, durability, light resistance, and light transmittance. 
     Thus, the hard coating film can find useful applications in various fields thanks to its excellent physical properties.

This application is a 35 U.S.C. § 371 National Phase Entry Applicationfrom PCT/KR2013/006778, filed on Jul. 29, 2013, and designating theUnited States, which claims priority under 35 U.S.C. § 119 to KoreanPatent Application No. 10-2012-0092532, filed on Aug. 23, 2012, andKorean Patent Application No. 10-2013-0089101, filed on Jul. 26, 2013,which are all hereby incorporated by reference in their entireties intothis application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a hard coating film. More particularly,the present invention relates to a hard coating film with high hardnessand impact resistance.

This application claims the benefit of Korean Patent Application No.10-2012-0092532, filed on Aug. 23, 2012, and Korean Patent ApplicationNo. 10-2013-0089101, filed on Jul. 26, 2013, which are all herebyincorporated by reference in their entireties into this application.

2. Description of the Related Art

With the advance of mobile appliances such as smart phones, tablet PCsand the like, substrates for displays have recently been required tobecome lighter and slimmer. Display windows or front panels of suchmobile appliances are generally made of glass or reinforced glass bothof which have excellent mechanical properties. However, glass suffersfrom the disadvantage of being heavy and being easily broken by anexternal impact.

As an alternative to glass, plastic resin films have emerged. Theirlight weight and resistance to impact are consistent with the trend ofpursuing lighter and slimmer mobile appliances. Particularly, a filmwith high hardness and wear resistance is required. In this regard, itis proposed to utilize a structure in which the substrate is coated witha hard coating layer.

First of all, increasing the thickness of the hard coating layer isconsidered as an approach to improving the surface hardness thereof. Infact, the hard coating layer should be of a minimal thickness to ensurethe surface hardness of the hard coating layer. As the hard coatinglayer increases in thickness, the surface hardness thereof may becomehigher. However, a thicker hard coating layer, although increasing thesurface hardness, is more prone to setting shrinkage which leads towrinkling or curling with the concomitant production of cracks orexfoliations, and thus thick hard coating layers are difficult to employin practice.

Recently, some methods have been proposed for conferring a high hardnesson hard coating films, without the problems of cracking and settingshrinkage-induced curling.

Korean Patent Application Publication No. 2010-0041992 discloses a hardcoating film composition, free of monomers, comprising a binder resinbased on ultraviolet-curable polyurethane acrylate oligomers. However,this hard coating film has a pencil hardness of about 3H, and thus thestrength thereof is not sufficient to be a substitute for glass panelsfor displays.

SUMMARY OF THE INVENTION

Accordingly, the present invention has been made keeping in mind theabove problems occurring in the prior art, and is to provide a hardcoating film which exhibits high hardness and impact resistance.

In accordance with an aspect thereof, the present invention provides ahard coating film, comprising:

a supporting substrate;

a first hard coating layer, formed on one side of the supportingsubstrate, having a first elastic modulus; and

a second hard coating layer, formed on another side of the supportingsubstrate, having a second elastic modulus, with a difference betweenthe first and the second elastic moduli set to be 500 MPa or greater.

Characterized by high hardness, impact resistance, scratch resistance,and transparency, the hard coating film of the present invention can beusefully applied to touch panels of mobile terminals, smart phones ortablet PCs, and as a cover or device panel for various displays.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention addresses a hard coating film, comprising:

a supporting substrate;

a first hard coating layer, formed on one side of the supportingsubstrate, having a first elastic modulus; and

a second hard coating layer, formed on another side of the supportingsubstrate, having a second elastic modulus, with a difference betweenthe first and the second elastic moduli set to be 500 MPa or greater.

All of the terms used in the specification are taken only to illustrateembodiments, and are not intended to limit the present invention. Asused herein and in the appended claims, the singular forms “a”, “an”,and “the” include plural reference unless the context clearly dictatesotherwise.

Additionally, the word “on” or “above,” as used in the context offormation or construction of one element, means pertaining to the directformation or construction of one element on another element directly orthe additional formation or construction of one element between layersor on a subject or substrate.

The above detailed descriptions of embodiments of the invention are notintended to be exhaustive or to limit the invention to the precise formdisclosed above. While specific embodiments of, and examples for theinvention are described above for illustrative purposes, variousequivalent modifications are possible within the scope of the invention,as those skilled in the relevant art will recognize.

These and other non-limiting characteristics of the disclosure are moreparticularly disclosed below

In accordance with an aspect thereof, the present invention provides ahard coating film, comprising: a supporting substrate; a first hardcoating layer, formed on one side of the supporting substrate, having afirst elastic modulus; and a second hard coating layer, formed onanother side of the supporting substrate, having a second elasticmodulus, with a difference between the first and the second elasticmoduli set to be 500 MPa or greater.

So long as it is transparent, any plastic resin, whether capable ofbeing stretched or not, may be used for the supporting substrate onopposite sides of which first and second hard coating layers are formed,respectively, without limitations imposed thereto. According to anembodiment of the present invention, the supporting substrate mayinclude, for example, a polyester such as polyethyleneterephtalate(PET), a polyethylene such as ethylene vinyl acetate (EVA), cyclicolefin polymer (COP), cyclic olefin copolymer (COC), polyacrylate (PAC),polycarbonate (PC), polyethylene (PE), polymethylmethacrylate (PMMA),polyetheretherketon (PEEK), polyethylenenaphthalate (PEN),polyetherimide (PEI), polyimide (PI), triacetylcellulose (TAC), MMA(methyl methacrylate), or a fluoro-polymer. The substrate may be asingle layer structure, and, if necessary, may be a multilayer structureincluding two or more layers composed of the same or differentmaterials, but is not particularly limited.

According to an embodiment of the present invention, the supportingsubstrate may be a multilayered substrate made ofpolyethyleneterephthalate (PET) or co-extruded polymethylmethacrylate(PMMA)/polycarbonate (PC).

Further, according to an embodiment of the present invention, thesupporting substrate may include a copolymer of polymethylmethacrylate(PMMA) and polycarbonate (PC).

The supporting substrate may range in thickness from 30 to 1,200 μm, orfrom 50 to 800 μm, but is not limited thereto.

As described above, the hard coating film of the present inventioncomprises a supporting substrate with first and second hard coatinglayers formed on opposite sides of the supporting substrate,respectively.

In the hard coating film of the present invention, the first hardcoating layer has a first elastic modulus while the second hard coatinghas a second elastic modulus, with a difference between the first andthe second elastic moduli set to be 500 MPa or greater.

Throughout the specification, “elastic modulus” means a value measuredaccording to ASTM D882.

That is, the hard coating film of the present invention comprises asupporting substrate sandwiched between two hard coating layers whichdiffer in elastic modulus from each other by 500 MPa or greater. A hardcoating layer with a higher elastic modulus exhibits high physicalstrength such as high hardness while the other coating layer with lowerelastic modulus has higher impact resistance and fold endurance. Hence,the hard coating film in which two hard coating layers with differentelastic moduli are deposited on opposite sides of a supportingsubstrate, respectively, is high in physical strength sufficient to be asubstitute for glass, and is much less prone to curling or cracking,thus guaranteeing high processability.

In one embodiment, the difference between the first and the secondelastic moduli is at least approximately 500 MPa, for example, rangesfrom approximately 500 to 3,000 MPa, or from approximately 500 to 2,500MPa, or from approximately 500 to 2,000 MPa.

In accordance with an embodiment of the present invention, the firstelastic modulus may be approximately 1,500 MPa or less, or may rangefrom approximately 300 to 1,500 MPa, from approximately 300 to 1,200MPa, or from approximately 300 to 1,000 MPa, while the second elasticmodulus may be approximately 2,000 MPa or greater, for example, rangesfrom approximately 2,000 to 3,500 MPa, from approximately 2,000 to 3,000MPa, or from approximately 2,000 to 2,800 MPa.

Given the conditions for the first and the second elastic moduli, thefirst hard coating layer may include a first photocurable crosslinkingcopolymer of a mono- to hexa-functional acrylate monomer and a firstphotocuarble elastic polymer.

As used herein, the term “acrylate” is intended to encompass acrylate,methancrylate, and derivatives thereof with various substituents.

As used herein, the term “photocurable elastic polymer” refers to apolymer which is of elasticity and contains a functional group thatundergoes UV light-triggered crosslink polymerization.

According to an embodiment of the present invention, the photocurableelastic polymer may have an elongation of approximately 15% or more, forexample, approximately 15 to 200%, approximately 20 to 200%, orapproximately 20 to 150%, as measured according to ASTM D638.

The first photocurable elastic polymer is crosslink-polymerized with themono- to hexa-functional acrylate monomer and then cured to form a firsthard coating layer, conferring flexibility and impact resistance to thehard coating film due to proper elastic modulus.

According to one embodiment of the present invention, the photocurableelastic polymer may be a polymer or oligomer having a weight averagemolecular weight of about 1,000 to about 600,000 g/mol or about 10,000to about 600,000 g/mol.

The photocurable elastic polymer may be at least one selected from thegroup consisting of polycaprolactone, a urethane acrylate polymer, andpolyrotaxane.

Among the polymers used as the first photocurable elastic polymer,polycaprolactone is formed by the ring-opening polymerization ofcaprolactone, and has excellent physical properties such as flexibility,impact resistance, durability and the like.

Retaining a urethane bond therein, a urethane acrylate polymer hasexcellent elasticity and durability.

A polyrotaxane is a polymer of rotaxane, a mechanically-interlockedmolecular architecture consisting of a dumbbell-shaped molecule which isthreaded through a cyclic moiety (macrocycle). The two components of arotaxane are kinetically trapped since the ends of the dumbbell(stoppers) are larger than the internal diameter of the ring and preventdisassociation of the components since this would require significantdistortion of the covalent bonds.

In one embodiment, the photocurable elastic polymer may include arotaxane comprising a cyclic moiety (macrocycle) in which lactonecompounds with a (meth)acrylate moiety conjugated to the end thereof arebonded each other; a thread moiety held within the macrocycle; andstoppers provided at both ends of the thread moiety so as to preventdissociation of the macrocycle.

No particular limitations are imposed on the macrocycle if it is largeenough to surround the linear moiety. The macrocycle may include afunctional group such as a hydroxide group, an amino group, a carboxylgroup, a thiol group, an aldehyde group or the like, which can reactwith other polymers or compounds. Specific examples of the macrocyclemay include α-cyclodextrin, β-cyclodextrin, γ-cyclodextrin and mixturesthereof.

Further, the thread may be used without limitation as long as it isgenerally linear in shape with a predetermined weight or greater.Preferably, a polyalkylene compound or a polylactone compound may beused in the thread. Specifically, a polyoxyalkylene compound includingan oxyalkylene repetitive unit of 1 to 8 carbon atoms or a polylactonecompound including a lactone repetitive unit of 3 to 10 carbon atoms maybe used in the thread.

Meanwhile, the stopper may be appropriately adjusted depending on thecharacteristics of the rotaxane compound to be prepared. For example,the stopper may be at least one selected from the group consisting of adinitrophenyl group, a cyclodextrin group, an amantane group, a trilylgroup, a fluorescein group and a pyrene group.

As the first hard coating layer containing the first photocurableelastic polymer is formed by photocuring, it is imparted with a properelastic modulus and allows the hard coating film to have high hardnessand flexibility, particularly ensuring excellent resistance to externalimpact.

Examples of the mono- to hexa-functional acrylate monomers may includehydroxyethylacrylate (HEA), hydroxyethylmethacrylate (HEMA),hexanedioldicrylate (HDDA), tripropylene glycol dicrylate (TPGDA),ethylene glycol dicrylate (EGDA), trimethylolpropane triacrylate(TMPTA), trimethylolpropaneethoxy triacrylate (TMPEOTA),glycerin-propoxylated triacrylate (GPTA), pentaerythritol tetraacrylate(PETA), dipentaerythritol hexaacrylate (DPHA), and the like. These tri-to hexa-functional acrylate monomers may be used alone or incombination.

The first photocurable crosslinking copolymer may be a copolymer inwhich the first photocurable elastic polymer is crosslinked with a mono-to hexa-functional acrylate monomer.

Based on 100 weight parts thereof, the first photocurable crosslinkingcopolymer may contain the first photocurable elastic polymer in anamount of approximately 20 to 80 weight parts and the mono- tohexa-functional acrylate monomer in an amount of 80 to 20 weight parts,or the first photocurable elastic polymer in an amount of approximately20 to 60 weight parts and the mono- to hexa-functional acrylate monomerin an amount of approximately 40 to 80 weight parts. Given the firstphotocurable crosslinking copolymer in which the first photocurableelastic polymer is crosslinked at such a high content, the first hardcoating film can be formed with a proper elastic modulus and exhibitsexcellent physical properties including impact resistance.

In one embodiment of the present invention, the first hard coating layeris 50 μm or more thick, for example, ranges in thickness fromapproximately 50 to 300 μm, from approximately 50 to 200 μm, or fromapproximately 50 to 150 μm, or from approximately 70 to 150 μm.

Meanwhile, the first hard coating layers may further include typicaladditives such as a surfactant, a yellowing inhibitor, a leveling agent,an antifouling agent and the like in addition to the above-mentionedcomponents. Here, the content of the additive is may be variouslyadjusted to the degree that the physical properties of the hard coatingfilm are not degraded. Its content is not particularly limited, butpreferably ranges from approximately 0.1 to 10 weight parts, based on100 weight parts of the first photocurable crosslinking copolymer.

According to an embodiment of the present invention, for example, thefirst hard coating layer may include a surfactant as an additive. Thesurfactant may be a mono- or bi-functional fluorine acrylate, a fluorinesurfactant, or a silicon surfactant. In this context, the surfactant maybe contained in a dispersed or crosslinked form in the photocurablecrosslinking copolymer. Further, a yellowing inhibitor may be used as anadditive. The yellowing inhibitor may be a benzophenone compound or abenzotriazole compound.

The first hard coating layer may be formed by photocuring a hard coatingcomposition comprising a mono- to hexa-functional acrylate monomer, afirst photocurable elastic polymer, a solvent, and optionally anadditive in the presence of a photoinitiator after the coatingcomposition is applied to the substrate.

Examples of the photoinitiator may include, but are not limited to,1-hydroxy-cyclohexyl-phenyl ketone,2-hydroxy-2-methyl-1-phenyl-1-propanone,2-hydroxy-1-[4-(2-hydroxyethoxyl)phenyl]-2-methyl-1-propanone,methylbenzoylformate, α,α-dimethoxy-α-phenylacetophenone,2-benzoyl-2-(dimethylamino)-1-[4-(4-morpholinyl)phenyl]-1-butanone,2-methyl-1-[4-(methylthio)phenyl]-2-(4-morpholinyl)-1-propanone,diphenyl(2,4,6-trimethylbenzoyl)-phosphine oxide,bis(2,4,6-trimethylbenzoyl)-phenylphophine oxide, and the like. Further,the photoinitiator may be commercially available, such as those soldunder brand name, Irgacure 184, Irgacure 500, Irgacure 651, Irgacure369, Irgacure 907, Darocur 1173, Darocur MBF, Irgacure 819, Darocur TPO,Irgacure 907, and Esacure KIP 100F. These photoinitiators may be usedalone or in combination.

Examples of the organic solvent available in the present invention mayinclude: alcohols such as methanol, ethanol, isopropyl alcohol, butanoland the like; alkoxy alcohols such as 2-methoxy ethanol, 2-ethoxyethanol, 1-methoxy-2-propanol and the like; ketones such as acetone,methyl ethyl ketone, methyl isobutyl ketone, methyl propyl ketone,cyclohexanone and the like; ethers such as propyleneglycol monopropylether, propyleneglycol monomethyl ether, ethyleneglycolmonoethyl ether,ethyleneglycol monopropyl ether, ethyleneglycol monobutyl ether,diethyleneglycol monomethyl ether, diethyleneglycol monoethyl ether,diethyleneglycol monopropyl ether, diethyleneglycol monobutyl ether,diethyleneglycol-2-ethylhexyl ether and the like; and aromatic solventssuch as benzene, toluene, xylene and the like. These organic solventsmay be used alone or in combination.

In the composition for the first hard coating layer (hereinafterreferred to as “first hard coating composition”), the solid fractionincluding the mono- to hexa-functional acrylate monomer, the firstphotocurable elastic polymer, the photoinitiator, and other additivesmay be used at a weight ratio of about 70:30 to about 99:1 with regardto the solvent. As such, when the hard coating composition has a highsolid content, the first hard coating composition increases inviscosity, and thus can allow for thick coating, for example, form thefirst hard coating layer at a thickness of 50 μm or more.

So long as it meets the conditions for the second elastic modulus setforth above, the components involved in the second hard coating layerare not specifically limited. According to one embodiment, the secondhard coating layer may contain a second photocurable crosslinkingcopolymer of a tri- to hexa-functional acrylate monomer and a secondphotocurable elastic polymer, and organic microparticles dispersed inthe second photocurable crosslinking copolymer.

Among the tri- to hexa-functional acrylate monomers aretrimethylolpropane triacrylate (TMPTA), trimethylolpropaneethoxytriacrylate (TMPEOTA), glycerin-propoxylated triacrylate (GPTA),pentaerythritol tetraacrylate (PETA), dipentaerythritol hexaacrylate(DPHA), and the like. These tri- to hexa-functional acrylate monomersmay be used alone or in combination.

According to one embodiment of the present invention, the secondphotocurable elastic polymer may have an elongation of approximately 15%or more, for example, approximately 15 to 200%, approximately 20 to200%, or approximately 20 to 150%, as measured according to ASTM D638.

According to another embodiment of the present invention, thephotocurable elastic polymer may be a polymer or oligomer having aweight average molecular weight of about 1,000 to about 600,000 g/mol orabout 10,000 to about 600,000 g/mol.

The second photocurable elastic polymer may be the same as or differentfrom the first photocurable elastic polymer, and the description of thefirst hard coating layer is true of the second photocurable elasticpolymer.

In one embodiment, the second photocurable elastic polymer may be atleast one selected from the group consisting of polycaprolactone, aurethane acrylate polymer, and polyrotaxane. When the secondphotocurable elastic polymer is photocured, the resulting second hardcoating layer is imparted with high resistance against external impact.

The second photocurable crosslinking copolymer contained in the secondhard coating layer may be a copolymer in which a tri- to hexa-functionalacrylate monomer is crosslinked with the second photocurable elasticpolymer.

Based on 100 weight parts thereof, the second photocurable crosslinkingcopolymer may contain the second photocurable elastic polymer in anamount of 5 to 20 weight parts and the tri- to hexafunctional acrylatemonomer in an amount of 80 to 95 weight parts. When the tri- tohexafunctional acrylate monomer and the second photocurable elasticpolymer are crosslink polymerized with each other at the weight ratio,the second hard coating layer has an elastic modulus in a suitable rangeand exhibits high hardness and impact resistance without deterioratingother properties including curling or light resistance.

According to one embodiment, the second hard coating layer may compriseinorganic microparticles dispersed in the second photocurablecrosslinking copolymer.

According to another embodiment of the present invention, the inorganicparticles may be inorganic nanoparticles having a particle size of about100 nm or less, about 10 to about 100 nm or about 10 to about 50 nm. Forexample, silica particles, aluminum oxide particles, titanium oxideparticles or zinc oxide particles may be used as the inorganicparticles.

The inorganic particles in the hard coating layer make additionalcontribution to improving the hardness of the hard coating film.

Based on 100 weight parts thereof, the second hard coating layer maycontain the second photocurable crosslinking copolymer in an amount ofapproximately 40 to 90 weight parts and the inorganic microparticles inan amount of 10 to 60 weight parts, or the second photocurablecrosslinking copolymer in an amount of approximately 50 to 80 weightparts and the inorganic microparticles in an amount of approximately 20to 50 weight parts. Given the amounts of the photocurable copolymer andthe inorganic microparticles within the ranges set forth above, the hardcoating film can be formed with excellent physical properties.

After being completely cured, the second hard coating layer may have athickness of approximately 50 to 300 μm, approximately 50 to 200 μm,approximately 50 to 150 μm, or approximately 70 to 150 μm.

Meanwhile, the second hard coating layer may further include typicaladditives such as a surfactant, a yellowing inhibitor, a leveling agent,an antifouling agent and the like in addition to inorganicmicroparticles. Here, the content of the additive is may be variouslyadjusted to the degree that the physical properties of the hard coatingfilm are not degraded. Its content is not particularly limited, butpreferably ranges from approximately 0.1 to 10 weight parts, based on100 weight parts of the second photocurable crosslinking copolymer.

According to an embodiment of the present invention, for example, thesecond hard coating layer may include a surfactant as an additive. Thesurfactant may be a mono- or bi-functional fluorine acrylate, a fluorinesurfactant, or a silicon surfactant. In this context, the surfactant maybe contained in a dispersed or crosslinked form in the secondphotocurable crosslinking copolymer. Further, the second hard coatinglayer may include a yellowing inhibitor as an additive. The yellowinginhibitor may be a benzophenone compound or a benzotriazole compound.

The second hard coating layer may be formed by photocuring a second hardcoating composition comprising a tri- to hexa-functional acrylatemonomer, a second photocurable elastic polymer, inorganicmicroparticles, optionally an additive and an organic solvent in thepresence of a photoinitiator after the second composition is applied tothe other side of the substrate.

The photoinitiator may be the same as or different from that used in thefirst hard coating composition. For details of the photoinitiator,reference may be made to the description of the first hard coatingcomposition. The photoinitiators may be used alone or in combination.

Also, the organic solvent may be the same as or different from that usedfor the first hard coating composition. For details of the solvent,reference may be made to the description of the first coatingcomposition.

In accordance with one embodiment, the hard coating film of the presentinvention may further comprise at least one layer, or film, such as aplastic resin film, an adhesive film, an releasable film, anelectrically conductive film, an electrically conductive layer, acurable resin layer, a non-conductive film, a metal mesh layer, orpatterned metal layer on either the first or the second hard coatinglayer. In addition, the layer, or film may take any form such as amonolayer, a bilayer or a lamination. The layer, or film may beconstructed on the hard coating layer by, but not limited to, laminatinga freestanding film with the aid of an adhesive or an adhesive film, orby coating, deposition, or sputtering.

Particularly, the layer, or film may be brought into direct contact withthe first hard coating layer to allow the hard coating film to improvein resistance against external impact and abrasion.

To increase adhesiveness with the layer, or film, the first hard coatinglayer may be surface treated with plasma, corona discharge, or analkaline solution of sodium hydroxide or potassium hydroxide.

The first and the second hard coating compositions may be applied ontothe front and back sides of the supporting substrate, respectively, in asequential or simultaneous manner before photocuring.

The hard coating film according to the present invention may be preparedin the following manner.

First, the first hard coating composition containing the above-mentionedcomponents is applied to one side of the supporting substrate andphotocured to form a first hard coating layer.

Any method that is available in the art would be used in the applicationof the first hard coating composition without particular limitations.For example, the hard coating composition may be applied by bar coating,knife coating, roll coating, blade coating, die coating, micro-gravurecoating, comma coating, slot die coating, lip coating, solution castingor the like.

Next, the applied first hard coating composition is photocured under UVlight to form a first hard coating layer.

UV radiation may be emitted at a dose of approximately 20 to 600 mJ/cm²,or approximately 50 to 500 mJ/cm². Any light source that is used in theart would be applied to the present invention without particularlimitation. For example, a high-pressure mercury lamp, a metal halidelamp, a black light fluorescent lamp or the like may be used. Thephotocuring may be carried out by irradiating UV light at the dose forabout 30 sec to about 15 min, or for about 1 to about 10 min.

After being completely cured, the first hard coating layer may have athickness of approximately 50 to 300 μm, approximately 50 to 200 μm,approximately 50 to 150 μm, or approximately 70 to 150 μm.

The presence of the first hard coating layer ensures the high hardnessof the hard coating film without a decrease in curling property.

Subsequently, the second hard coating composition comprising theabove-mentioned components is applied to the other side, e.g., back sideof the supporting substrate. Then, the second hard coating compositionis cured to form a second hard coating layer after exposure to UV light.During the photocuring of the second hard coating composition, UV lightis irradiated to a side opposite to that coated with the first hardcoating composition. Thus, the curl which may be generated by settingshrinkage in the former photocuring step is counterbalanced to afford aflat hard coating film. No additional flattering processes are thusneeded.

UV radiation may be emitted at a dose of approximately 20 to 600 mJ/cm²,or approximately 50 to 500 mJ/cm². Any light source that is used in theart would be applied to the present invention without particularlimitation. For example, a high-pressure mercury lamp, a metal halidelamp, a black light fluorescent lamp or the like may be used. Thephotocuring may be carried out by irradiating UV light at the dose forabout 30 sec to about 15 min, or for about 1 to about 10 min.

After being completely cured, the second hard coating layer may have athickness of approximately 50 to 300 μm, approximately 50 to 200 μm,approximately 50 to 150 μm, or approximately 70 to 150 μm.

In the preparation method of the hard coating film according to oneembodiment, coating and photocuring steps of the first hard coatingcomposition may be performed in advance of or following those of thesecond hard coating composition.

For use as a cover for mobile terminals or tablet PCs, the hard coatingfilm must have hardness or impact resistance elevated sufficiently to bea substitute for glass. Even when formed at a high thickness on thesubstrate, the hard coating layer of the present invention is less proneto curling or cracking, and imparts the hard coating film with hightransparency and impact resistance.

The hard coating film according to the present invention is superior inhardness, scratch resistance, transparency, durability, lightresistance, and light transmittance.

The impact resistance of the hard coating film is high enough to be asubstitute for glass. For example, the hard coating film of the presentinvention may not crack even after a steel bead weighing 22 g is freelydropped ten times from a height of 50 cm thereto.

In addition, the second hard coating layer in the hard coating film ofthe present invention may have a pencil hardness of 7H or more, 8H ormore, or 9H or more at a load of 1 kg.

Further, after the second hard coating layer in the hard coating film ofthe present invention is tested by double rubbing 400 times with a steelwool #0000 under a load of 500 g on a friction tester, only two or lessscratch may appear.

The hard coating film of the present invention may have a lighttransmittance of 91.0% or more, or 92.0% or more, and a haze of 1.0% orless, 0.5% or less, or 0.4% or less.

Further, the hard coating film of the present invention may have aninitial color b value of 1.0 or less. After the hard coating film isexposed to UV-B under an ultraviolet lamp for 72 hrs or more, it mayhave a color b* value which differs from the pre-exposed color b* valueby 0.5 or less, or by 0.4 or less.

When the hard coating film of the present invention is disposed on aplane after exposure to a temperature of 50° C. or higher at a humidityof 80% or higher for 70 hrs, the maximum distance at which each edge orside of the hard coating film is spaced apart from the plane may beabout 1.0 mm or less, about 0.6 mm or less, or about 0.3 mm or less.More particularly, when the hard coating film of the present inventionis disposed on a plane after exposure to a temperature of 50° C. to 90°C. at a humidity of 80% to 90% for 70 to 100 hrs, each edge or side ofthe hard coating film is spaced apart from the plane by about 1.0 mm orless, about 0.6 mm or less, or about 0.3 mm or less, maximally.

Exhibiting excellent physical properties including hardness, impactresistance, scratch resistance, transparency, durability, lightresistance, and light transmittance, the hard coating film of thepresent invention has useful applications in various fields. Forexample, the hard coating film of the present invention can be used inthe touch panels of mobile terminals, smart phones or tablet PCs, and ascovers or device panels for various displays

A better understanding of the present invention may be obtained throughthe following examples which are set forth to illustrate, but are not tobe construed as limiting the present invention.

EXAMPLES Preparation Example 1 Preparation of Photocurable ElasticPolymer

In a reactor, 50 g of a caprolactone-grafted polyrotaxane polymer[A1000, Advanced Soft Material INC] was mixed with 4.53 g of Karenz-AOI[2-acryloylethyl isocyanate, Showadenko Inc.], 20 mg of dibutyltindilaurate [DBTDL, Merck Corp.], 110 mg of hydroquinone monomethyleneether, and 315 g of methyl ethyl ketone. Then, the mixture was reactedat 70° C. for 5 hrs to obtain polyrotaxane in which polylactone with anacrylate moiety conjugated to the end thereof acted as the macrocyclewhile cyclodextrin was positioned as the stopper.

The polyrotaxane had a weight average molecular weight of 600,000 g/mol,and was found to have an elongation of 20%, as measured according toASTM D638.

Example 1

A first hard coating composition was prepared by mixing 6 g oftrimethylolpropane triacrylate (TMPTA), 4 g of the polyrotaxane preparedin Preparation Example 1, 0.2 g of a photoinitiator (brand name: DarocurTPO), 0.1 g of a benzotriazole-based yellowing inhibitor (brand name:Tinuvin 400), 0.05 g of a fluorine surfactant (brand name: FC4430), and1 g of methylethylketone.

A second hard coating composition was prepared by mixing 9 g of asilica-dipentaerythritolhexacrylate (DPHA) composite in which silicananoparticles with a particle size of 20˜30 nm were dispersed by 40 wt %(silica 3.6 g, DPHA 5.4 g), 1 g of the polyrotaxane of PreparationExample 1, 0.2 g of a photoinitiator (brand name: Darocur TPO), 0.1 g ofa benzotriazole-based yellowing inhibitor (brand name: Tinuvin 400), and0.05 g of a fluorine surfactant (brand name: FC4430).

The second hard coating composition was applied to a PET substrate 188μm thick with a size of 15 cm×20 cm, and then subjected to firstphotocuring by exposure to 280-350 nm UV light from a black lightfluorescence lamp.

Subsequently, the first hard coating composition was applied to the backside of the substrate, and then exposed to 280-350 nm UV light from ablack light fluorescence lamp to give a hard coating film. Each of thefirst and the second hard coating layers formed on both sides of thesubstrate was 100 μm thick.

Example 2

A hard coating film was prepared in the same manner as in Example 1,with the exception that 4 g of a urethane acrylate polymer (brand name:UA200PA, Shinnakamura Chemicals Corp., weight average molecular weight:2,600 g/mol, elongation measured according to ASTM D638: 170%), and 1 gof a urethane acrylate polymer (brand name: UA200PA) were used, insteadof 4 g and 1 g of the polyrotaxane of Preparation Example 1 in the firstand the second hard coating compositions, respectively.

Example 3

A hard coating film was prepared in the same manner as in Example 1,with the exception that 4 g of a urethane acrylate polymer (brand name:UA340PA, Shinnakamura Chemicals Corp., weight average molecular weight:13,000 g/mol, elongation measured according to ASTM D638: 150%), and 1 gof a urethane acrylate polymer (brand name: UA340PA) were used, insteadof 4 g and 1 g of the polyrotaxane of Preparation Example 1 in the firstand the second hard coating compositions, respectively.

Example 4

A hard coating film was prepared in the same manner as in Example 1,with the exception that 1 g of a urethane acrylate polymer (brand name:UA200PA) was used, instead of 1 g of the polyrotaxane of PreparationExample 1 in the second hard coating composition.

Example 5

A hard coating film was prepared in the same manner as in Example 1,with the exception that 1 g of a urethane acrylate polymer (brand name:UA340PA) was used, instead of 1 g of the polyrotaxane of PreparationExample 1 in the second hard coating composition.

Example 6

A hard coating film was prepared in the same manner as in Example 1,with the exception that each of the first and the second hard coatinglayers formed on both sides of the substrate was 150 μm after completionof the photocuring.

Example 7

A first hard coating composition was prepared by mixing 5 g oftrimethylolpropane triacrylate (TMPTA), 5 g of the polyrotaxane preparedin Preparation Example 1, 0.2 g of a photoinitiator (brand name: DarocurTPO), 0.1 g of a benzotriazole-based yellowing inhibitor (brand name:Tinuvin 400), 0.05 g of a fluorine surfactant (brand name: FC4430), and1 g of methylethylketone.

A second hard coating composition was prepared by mixing 9 g of asilica-dipentaerythritolhexacrylate (DPHA) composite in which silicananoparticles with a particle size of 20˜30 nm were dispersed by 40 wt %(silica 3.6 g, DPHA 5.4 g), 1 g of the polyrotaxane of PreparationExample 1, 0.2 g of a photoinitiator (brand name: Darocur TPO), 0.1 g ofa benzotriazole-based yellowing inhibitor (brand name: Tinuvin 400), and0.05 g of a fluorine surfactant (brand name: FC4430).

The first hard coating composition was applied to a PET substrate 100 μmthick with a size of 15 cm×20 cm, and then subjected to firstphotocuring by exposure to 280-350 nm UV light from a black lightfluorescence lamp. After completion of the photocuring, the first hardcoating layer thus formed was 125 μm thick.

The second hard coating composition was applied to the first hardcoating layer, and subjected to second photocuring by exposure to280-350 nm UV light from a black light fluorescence lamp to give a hardcoating film. The second hard coating layer laminated on the first hardcoating was 100 μm thick.

Comparative Example 1

A first hard coating composition was prepared by mixing 10 g oftrimethylolpropane triacrylate (TMPTA), 0.2 g of a photoinitiator (brandname: Darocur TPO), 0.1 g of a benzotriazole-based yellowing inhibitor(brand name: Tinuvin 400), 0.05 g of a fluorine surfactant (brand name:FC4430), and 1 g of methylethylketone.

A second hard coating composition was prepared by mixing 10 g of asilica-dipentaerythritolhexacrylate (DPHA) composite in which silicananoparticles with a particle size of 20˜30 nm were dispersed by 40 wt %(silica 4 g, DPHA 6 g), 0.1 g of a benzotriazole-based yellowinginhibitor (brand name: Tinuvin 400), and 0.05 g of a fluorine surfactant(brand name: FC4430).

The second hard coating composition was applied to a PET substrate 188μm thick with a size of 15 cm×20 cm, and then subjected to firstphotocuring by exposure to 280-350 nm UV light from a black lightfluorescence lamp.

Subsequently, the first hard coating composition was applied to the backside of the substrate, and then exposed to 280-350 nm UV light from ablack light fluorescence lamp to give a hard coating film. Each of thefirst and the second hard coating layers formed on both sides of thesubstrate was 100 μm thick.

Elastic moduli of the hard coating films prepared in Examples 1 to 7 andComparative Example 1 are summarized in Table 1, below.

TABLE 1 Elastic Modulus of 1^(st) Elastic Modulus of 2^(nd) Hard CoatingLayer Hard Coating Layer Difference (unit: MPa) (uni: MPa) (unit: MPa)Ex. 1 600 2500 1900 Ex. 2 550 2300 1750 Ex. 3 400 2350 1950 Ex. 4 6002300 1700 Ex. 5 600 2350 1750 Ex. 6 600 2500 1900 Ex. 7 350 2500 2150 C.Ex. 1 3100 3300 200

Test Examples Measuring Method

1) Pencil Hardness

Pencil hardness was evaluated according to the Japanese Standard JISK5400. In this regard, a pencil hardness meter was reciprocated threetimes on the second hard coating layer of each of the hard coating filmsunder a load of 1.0 kg to determine the hardness at which no scratchesappeared.

2) Scratch Resistance

After being loaded to a friction tester, a steel wool (#0000) wasreciprocated 400 times on the second hard coating layer of each of thehard coating films under a load of 0.5 kg, and scratches thus formedwere counted. Evaluation was made of the scratch resistance of the filmsby marking 0 for two or less scratches, for two to less than fivescratches, and x for five or more scratches.)

3) Light Resistance

Differences in color b* value of the hard coating films were measuredbefore and after exposure to UVB from UV lamp for 72 hrs.

4) Transmittance and Haze

The hard coating films were measured for transmittance and haze using aspectrophotometer (brand name: CHO-400))

5) Curl Property at High Humidity and Temperature

After a hard coating film piece with dimensions of 10 cm×10 cm wasstored for 72 hrs in a chamber maintained at a temperature of 85° C. anda humidity of 85%, it was placed on a flat plane. A maximal distance atwhich each edge of the piece was apart from the plane was measured.

6) Cylindrical Bending Test

Each of the hard coating films was wound on a cylindrical mandrel havinga diameter of 1 cm so that the first hard coating layer is towardoutside. When the hard coating film was not cracked, it was evaluated asOK. If the hard coating film was cracked, it was evaluated as X.

7) Impact Resistance

The impact resistance of the second hard coating layer of each of thehard coating films was evaluated by determining whether or not each ofthe hard coating films was cracked when a 22 g steel ball was freelydropped 10 times thereon from a height of 50 cm. Each of the hardcoating films was evaluated as OK when it was not cracked, and as X whencracked.

The results of the physical properties measured in each of the hardcoating films are summarized in Table 2 below.

TABLE 2 C. Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 7 Ex. 1 Pencilhardness 8H 9H 8H 9H 8H 9H 9H 9H Scratch resistance ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯Light resistance 0.20 0.18 0.25 0.20 0.23 0.26 0.21 0.42 Transmittance92.0 92.2 92.0 92.5 92.2 92.0 92.1 92.1 Haze 0.3 0.3 0.2 0.2 0.4 0.4 0.30.2 Bending test OK OK OK OK OK OK OK X Curl property at high 0.3 mm 0.2mm 0.3 mm 0.2 mm 0.4 mm 0.4 mm 0.2 mm 0.4 mm humidity & temperatureImpact resistance OK OK OK OK OK OK OK X

As shown in Table 2 above, it can be ascertained that all of the hardcoating films in which the second hard coating layer is greater inelastic modulus than the first hard coating layer by 500 MPa or largerexhibit good physical properties. In contrast, the film, prepared inComparative Example 1, in which the first and the second hard coatinglayers differ in elastic modulus by 500 MPa or less was found to beinsufficient in impact resistance.

What is claimed is:
 1. A hard coating film, comprising: a supportingsubstrate; a first hard coating layer, formed on one side of thesupporting substrate, having a first elastic modulus; and a second hardcoating layer, formed on another side of the supporting substrate,having a second elastic modulus, with a difference between the first andthe second elastic moduli set to be 500 MPa or greater, wherein the hardcoating film exhibits a pencil hardness of 7H or more at a load of 1 kg.2. The hard coating film of claim 1, wherein the difference between thefirst and the second elastic moduli set to range from 500 to 3,000 MPa.3. The hard coating film of claim 1, wherein the first elastic modulusis 1,500 MPa or less and the second elastic modulus is 2,000 MPa ormore.
 4. The hard coating film of claim 1, wherein the first hardcoating layer comprises a first photocurable crosslinking copolymer of afirst photocurable elastic polymer and a mono to hexa-functionalacrylate monomer, and the second coating layer comprises a secondphotocurable crosslink copolymer of a second photocurable elasticpolymer and a tri- to hexa-functional acrylate monomer, and inorganicmicroparticles dispersed in the second photocurable crosslinkingcopolymer.
 5. The hard coating film of claim 4, wherein the first andthe second photocurable elastic polymers are the same or different, andindependently have an elongation of 15 to 200%, as measured according toASTM
 0638. 6. The hard coating film of claim 4, wherein the first andthe second photocurable elastic polymers are the same o different, andindependently comprise at least one selected from the group consistingof a polycaprolactone, a urethane acrylate polymer, and a polyrotaxane.7. The hard coating film of claim 6, wherein the polyrotaxane comprisesa cyclic moiety (macrocycle) in which lactone compounds with a (meth)acrylate moiety conjugated to the end thereof are bonded to each other;a thread moiety held within the macrocycle; and stoppers provided atboth ends of the thread moiety so as to prevent dissociation of themacrocycle.
 8. The hard coating film of claim 4, wherein the first hardcoating layer contains the first photocurable elastic polymer in anamount of 20 to 80 weight parts and the mono- to hexa-functionalacrylate monomer in an amount of 80 to 20 weight parts, based on 100weight parts of the first hard coating layer.
 9. The hard coating filmof claim 4, wherein the second hard coating layer contains 5 to 20weight parts of the second photocurable elastic polymer polymerized with80 to 95 weight parts of the tri- to hexa-functional acrylate monomer,based on 100 weight parts of the second photocurable crosslinkingcopolymer.
 10. The hard coating film of claim 4, wherein the second hardcoating layer contains the second photocurable crosslinking copolymer inan amount of 40 to 90 weight parts, and the inorganic microparticles inan amount of 10 to 60 weight parts, based on 100 weight parts of thesecond hard coating layer.
 11. The hard coating film of claim 4, whereinthe mono- to hexa-functional acrylate monomer is selected from the groupconsisting of hydroxyethylacrylate (REA), hydroxyethylmethacrylate(HEMA), hexandiol dicrylate (HDDA), tripropylene glycol dicrylate(TPGDA), ethylene glycol dicrylate (EGDA), trimethylolpropanetriacrylate (TMPTA), trimethylolpropane ethoxytriacrylate (TMPEOTA),glycerinpropoxylated triacrylate (GPTA), pentaerythritol tetraacrylate(PETA), dipentaerythritol hexaacrylate (DPHA), and a combinationthereof.
 12. The hard coating film of claim 4, wherein the tri- tohexa-functional acrylate monomer is selected from the group consistingof trimethylolpropane triacrylate (TMPTA), trimethylolpropaneethoxytriacrylate (TMPEOTA), glycerin propoxylated triacrylate (GPTA),pentaerythritol tetraacrylate (PETA), dipentaerythritol hexaacrylate(DPHA), and a combination thereof.
 13. The hard coating film of claim 1,wherein the substrate includes at least one selected frompolyethyleneterephtalate (PET), ethylene vinyl acetate (EVA), cyclicolefin polymer (COP), cyclic olefin copolymer (COC), polyacrylate (PAC),polycarbonate (PC), polyethylene (PE), polymethylmethacrylate (PMMA),polyetheretherketon (PEEK), polyethylenenaphthalate (PEN),polyetherimide (PEI), polyimide (PI), triacetylcellulose (TAC), MMA(methyl methacrylate), and a fluoro-polymer.
 14. The hard coating filmof claim 1, wherein the first and the second hard coating layersindependently range in thickness from 50 to 300 μm.
 15. The hard coatingfilm of claim 1, wherein the hard coating film does not crack when a 22g steel ball is freely dropped 10 times thereon from a height of 50 cm.16. The hard coating film of claim 1, wherein two or less scratches areformed when the surface of the hard coating layer is rubbed byreciprocating a steel wool #0000 thereon 400 times under a load of 500g.
 17. The hard coating film of claim 1, having a light transmittance of91% or more, a haze of 0.4 or less, and a color b* value of 1.0 or less.18. The hard coating film of claim 1, wherein the hard coating film hasa color b* value after exposure to UV B for 72 hrs which differs from apre-exposed, color b* value by 0.5 or less.
 19. The hard coating film ofclaim 1, wherein when the hard coating film is disposed on a plane afterexposure to a temperature of 50° C. or higher at a humidity of 80% orhigher for 70 hrs or longer, each edge or side of the hard coating filmis spaced apart from the plane by 1.0 mm or less, maximally.
 20. Thehard coating film of claim 1, further comprising on the first or thesecond hard coating layer at least one layer selected from the groupconsisting of a plastic resin film, an adhesive film, an releasablefilm, an electrically conductive film, an electrically conductive layer,a curable resin layer, a non-conductive film, a metal mesh layer, and apatterned metal layer.